(1) Field of the Invention
The present invention relates to under carriages for supporting and displacing sliding doors at two spaced positions arranged to run on a trackrail embedded in concrete.
(2) Description of the Prior Art
Sliding doors, such as those used to close bombproof hangers, are extremely ponderous, each door half weighting about 90 tons for a length of 11 meters, for example. Each half-door of known sliding doors of this nature have run in each case via two so-called under carriages on a double-T rail set in concrete, having a height of, say, 60 cms, the track surface of which must be aligned horizontally with extreme precision. The carriages in these known systems comprise a rigid guiding system joined to the sliding door, with a roller chain circulating around the guiding system, comprising cylindrical rollers of a small diameter of approximately 5 cms, which imply a specially hardened surface of the track rail. The slightest deviation from the horizontal of the track rail surface and even minute soiling, e.g. sand and stones on the track rail, lead to highly different loads on the small track rollers which then fracture and, by leaving fragments on the track rail, also damage the other track rollers and thereby the under carriage as a whole. This may have the result that the displacement of the door halves becomes impossible. The resultant necessary dismantling of the carriages and substitution of new carriages is extremely difficult to perform. It has been attempted to prevent unequal loading on the rollers of the carriages by incorporating a neoprene layer between the door and the carriage, but is was observed that this plastics material or any analogous material begins to undergo flowage after a particular period, so that the aforesaid disadvantages arise again in an enhanced form.